Merge pull request #85777 from DougGregor/extern-global-variables-embedded-6.3

[6.3] [Embedded] Only strip "external" from global variables that have definitions

Author: DougGregor

lib/SIL/IR/Linker.cpp

@@ -522,7 +522,8 @@ void SILLinkerVisitor::visitGlobalAddrInst(GlobalAddrInst *GAI) {
   // In Embedded Swift, we want to actually link globals from other modules too,
   // so strip "external" from the linkage.
   SILGlobalVariable *G = GAI->getReferencedGlobal();
-  G->setLinkage(stripExternalFromLinkage(G->getLinkage()));
+  if (G->isDefinition())
+    G->setLinkage(stripExternalFromLinkage(G->getLinkage()));
 }
 
 //===----------------------------------------------------------------------===//

test/embedded/modules-extern.swift

@@ -21,8 +21,12 @@ public func publicFuncInAModule() {
 @usableFromInline
 internal func internalFuncInAModule() {
   some_c_api()
+  _ = globalVariable
 }
 
+@_extern(c)
+var globalVariable: Int
+
 // BEGIN Main.swift
 
 import MyModule